1. Field of the Invention
The present invention relates to expandable styrene type resin particles and molded foamed articles obtained from said particles. More particularly, it relates to styrene type resin foamed articles which exhibit excellent elasticity and impact resistance and can be conveniently used as a cushion packing material and a heat insulating material and also relates to the expandable styrene type resin particles (beads) which are used for the manufacture of said foamed article as well as to a method of manufacturing said foamed article.
The foamed article of a styrene type resin in accordance with the present invention can be especially preferably used in the form of a cushion packing material for various kinds of office automation machines, audio machines, domestic electric devices, etc.
2. Related Art
A foamed article of a styrene type resin in which expanded particles are fused to each other can be prepared by heating expandable styrene type resin beads containing a blowing agent with steam or the like and again heating the pre-expanded beads in a mold. However, the resulting foamed article of a styrene type resin has little elasticity and has a disadvantage of being easily damaged by impact. Therefore, its use is rather limited.
In general, the molded foamed articles of the polyolefin type, such as polyethylene, polypropylene, etc. have excellent impact resistance. However, in the case of the expandable polyolefin type resin particles used for the manufacture of such molded foamed article, a blowing agent which is impregnated therein is apt to easily escape and, therefore, it is necessary that they be promptly subjected to a pre-expansion to give the foamed particles. In addition, it is necessary for preparing a good foamed article to give an inner pressure by, for example, means of air to the pre-expanded particles and to mold them quickly before the inner pressure decreases or to fill them into a mold (which is able to enclose the pre-expanded particles in a compressed state to an extent of 30-50% and does not gas-tightly enclose them) followed by subjecting them to molding in the mold. For such a molding process, special auxiliary devices are necessary for the pre-expansion and the molding.
Under such circumstances, some attempts have been made for improving the impact resistance of the molded products by the use of a high impact polystyrene resin (hereinafter, referred to as an HIPS resin) which is an improved article of polystyrene resin with a butadiene rubber. For example, expandable resin particles using the HIPS resin are disclosed in the Japanese Patent Publications Sho 47/18428 and Sho 51/46536.
However, when those expandable styrene type resin particles are subjected to a pre-expanding immediately after having been taken out from a vessel in which a blowing agent is impregnated therein or from a polymerization impregnating vessel, the size of the cells in the pre-expanded particles are not uniform. Accordingly, when the expandable styrene type resin particles impregnated with a blowing agent are subjected to a pre-expanding, it is necessary to conduct an aging until the cells of the pre-expanded particles are made uniform so that the molding is possible. However, in the case of the above-mentioned conventional expandable styrene type resin particles, there is a problem in that they are usually stored and aged in a store house kept at a low temperature for as long as about twenty days.
Further, though the HIPS resin inherently greatly improves the impact resistance of the polystyrene particles, there is a problem that, when the HIPS resin is made into a molded foamed article, the impact resistance of the molded foamed article is hardly improved.
This is because, when the pre-expanding is carried out upon completion of the aging of said resin particles, the cells of the pre-expanded particles are very fine. The fine cells, i.e. the cells with very small diameters, are in a state such that their cell membranes are very thin and are easily damaged. Accordingly, they are broken at the time of the pre-expanding or further at the time of the molding due to applied vapor pressure whereby the foamed molded article with only a small proportion of closed cells (less than 50% of closed cells results.
The foamed molded article with a low proportion of closed cell foam is destroyed due to it having no efficient means of absorption of impact or, even if released from impact stress, it does not recover to the original shape and, when impact stress is applied repeatedly, the article cannot be used as a cushion packing material.
When a large amount of the polybutadiene component is further added in order to improve impact resistance, heat resistance is deteriorated and, upon molding, (partial) melting occurs on the outer part of the molded foamed article whereby it is not possible to afford a foamed article with satisfactory properties. Accordingly, there is a disadvantage that impact resistance is rather lowered.
In the Japanese Laid-Open Publications Sho-54/154471 and Sho-54/158467, there is a disclosure of expandable styrene type resin particles wherein a blowing agent is impregnated into a resin composition prepared by a mechanical mixing of a block copolymer of styrene/butadiene with a styrene type resin and, in the Japanese Laid-Open Publication Hei-03/182529, there is a disclosure of expandable resin particles wherein a blowing agent is impregnated into a resin composition prepared by a mechanical mixing of a block copolymer of a hydrogenated styrene/butadiene with an HIPS resin. It is noted that the foamed article prepared by foaming and molding those expandable resin particles exhibits some improved impact resistance as compared with the conventional foamed HIPS articles.
However, such block copolymers are considerably expensive and, therefore, the use of the block copolymer in large quantities is disadvantageous in terms of cost and its industrial application is difficult. In addition, the above-mentioned resin prepared by mixing the block copolymer with the HIPS resin is hard to foam highly and there is another disadvantage that a highly-foamed article with a density of 0.040 g/cm.sup.3 or less cannot be prepared. Moreover, the cells of the pre-expanded particles obtained from said resin are finer and, when they are made into a molded foamed article, the degree of fusion is worse whereby it is not possible to produce an article with good elasticity.